The present invention relates to processes for applying catalyst or catalyst support coatings onto ceramic supports. More particularly, the invention relates to methods for coating ceramic substrates with catalyst coatings wherein a pre-coating or passivation step is used to improve the properties of the catalyzed substrates, by reducing catalyst and/or support coating diffusion into the fine pore and microcrack structure of the substrates and to drying the passivation and catalyst coatings subsequent to application.
Recent attention has focused on basic improvements in the design and performance of ceramic wall-flow honeycomb filters for treating diesel exhaust gases to address tightening diesel engine emissions regulations being adopted in the United States and Europe. Design changes allowing for the use of catalyst coatings to control hydrocarbon and/or nitrogen oxide emissions are being implemented along with other improvements. The goal is to develop an improved high-temperature-resistant, high-thermal-shock-resistant, low cost honeycomb soot filter compatible with advanced emissions control catalyst technologies that can replace current high-cost and/or uncatalyzed particulate filters.
Among the filter designs being developed for this application are refractory ceramic oxide filters offering improved resistance to high exhaust temperatures encountered during decarbonizing filter regeneration cycles, as well as to the thermal shock conditions arising during rapid filter heat-up and cool-down in the course of startup and regeneration. Two compositions currently being employed for filter construction are cordierite and aluminum titanate. Examples of advanced cordierite and aluminum titanate compositions utilized within honeycomb filter designs being developed for these applications are disclosed in U.S. Pat. No. 6,541,407, entitled CORDIERITEBODY, U.S. Pat. No. 6,849,181, entitled MULLITE-ALUMINUM TITANATE DIESEL EXHAUST FILTER, and U.S. Pat. No. 6,620,751, entitled STRONTIUM FELDSPAR ALUMINUM TITANATE FOR HIGH TEMPERATURE APPLICATIONS, which are each herein incorporated by reference in their entirety. Other materials being employed for refractory, catalyst-compatible ceramic particulate filters are the refractory alkali zirconium phosphates as well as low-expansion alkali aluminosilicates such as beta-eucryptite and pollucite. Many of these same compositions, and other microcracked ceramic materials such as the calcium aluminates, are being considered for use as flow-through catalyst supports for the control of nitrogen oxide (NOx) emissions from automotive and diesel engines.
Advanced aluminum titanate ceramics are among the most promising candidates for use in diesel exhaust filter applications as these ceramics meet or exceed most specifications for high melting point, high thermal capacity, and low thermal expansion. However, one difficulty encountered with these and other porous ceramics intended to function as particulate filters is the need to maintain both high gas permeability and a low coefficient of thermal expansion throughout the processes involved in depositing catalysts on the filter walls. A general requirement is that a low average linear coefficient of thermal expansion (CTE) for these filters should be maintained. Preferably, increases in CTE resulting from the application of washcoats and catalysts should not exceed 10×10−7/° C. averaged over the range from 25-1000° C., and CTE values for the washcoated filters should not exceed 20×10−7/° C. over that same temperature range, in order to preserve the thermal shock resistance of the filter. Further, gas permeabilities through the catalyzed filter should be sufficient to maintain pressure drops below 8 kPa at exhaust gas space velocities up to 150,000 hr−1 after filter regeneration to remove trapped particulates.
A significant drawback associated with the application of the alumina or other washcoating materials typically employed to support the required emission control catalysts is a substantial increase in CTE and reductions in filter permeability. Present understanding is that during the washcoating or catalyzing process both wall porosity of the filter and the structural micro-cracks (crack widths of 0.1-3 microns) that are present in most of these ceramic materials are frequently filled with the washcoating material. This problem is most pronounced in the case of highly microcracked ceramics such as the aluminum titanates, particularly when the washcoating formulations contain materials of very fine particulate size (e.g., particle diameters in the 0.02-0.1 μm range), thereby facilitating the filling of the microcracks with the washcoating material.
Microcracking is a significant contributor to the low CTEs exhibited by many of these materials, with crack closure during heating considerably moderating the dimensional increases that would otherwise occur. As a result, the filling of these microcracks with washcoating constituents can result in some cases in much higher expansion coefficients e.g., in the range of 40-50×10−7/° C., in the washcoated structures. At these CTE levels the risk of structural damage to the filter under the normal conditions of exhaust filter use is unacceptable.
One approach to the problem of washcoat microcrack filling of conventional flow-through catalyst substrates for gasoline engine emissions control has been the use of so-called passivating coatings prior to the application of the associated catalyst. These passivating coatings are pre-coatings applied to the walls of the ceramic substrates prior to the washcoating that can block the washcoating materials from intruding into the microcrack structure of the ceramic. U.S. Pat. No. 4,532,228, entitled TREATMENT OF MONOLITHIC CATALYST SUPPORTS which is herein incorporated by reference in its entirety, provides some examples of coating materials that can be carbonized or otherwise solidified to provide a washcoat barrier, and then removed after the washcoat has been laid down.
To provide adequate protection against CTE increases in highly microcracked ceramics such as aluminum titanates while simultaneously providing an effective guard against unacceptable reductions in ceramic wall gas permeability certain polymeric barrier layers have been employed as the passivation coating. Specifically, these polymeric barrier layers are composed of mainly polymeric materials with hydrophobic and hydrophilic functional groups that are soluble and/or dispersible in a polar medium and that form a neutral or hydrophilic surface on the substrate, at least in the presence of acidic washcoating media. Examples of specific polymer types with these characteristics include ionene polymers, acid-activated aminoacrylate copolymers, and aliphatic acrylic acid copolymers. Another approach employed towards meeting the above-referenced requirements includes applying the passivation polymer barrier layer to the substrate composed of the hydrocarbon polymer, and thereafter over-coating the polymer barrier with an aqueous dispersion of a selected ceramic washcoating material to provide a ceramic-coated substrate. These approaches toward reducing CTE and maximizing the overall ceramic wall gas permeability are disclosed in U.S. patent application Ser. No. 10/641,638, filed Aug. 14, 2003, and entitled POROUS CERAMIC FILTER WITH CATALYST COATINGS, which is incorporated in its entirety herein by reference.
Heretofore, the production processes associated with the manufacturing of low CTE, high gas permeable filters employing the application of passivation and catalyst coatings noted above were time intensive and therefore costly procedures. Specifically, a lengthy drying process of 12 to 24 hours is typically employed subsequent to applying the passivation coating to the filter in order to remove water from the coating. The duration of this drying process is a significant factor with respect to how quickly the filters in process may be moved to the subsequent catalyzing step, and ultimately how quickly the overall process may be completed and filters delivered to the marketplace. Further, an additional drying step is accomplished subsequent to the application of the catalyst coating, thereby further increasing the time of the overall manufacturing cycle. Moreover, the drying processes currently utilized typically include the use of convection ovens that are relatively expensive to operate and maintain, and require a large production area to ensure adequate processing capabilities.
A method for applying and drying passivation coatings and catalyst coatings to ceramic supports and particularly diesel emission filters is desired that protects against CTE increases in highly microcracked ceramics while simultaneously providing an effective guard against unacceptable reductions in ceramic wall gas permeability. Further, the method should also decrease the overall manufacturing time of any given filter by substantially reducing the drying time as required in the process, and should moreover decrease the overall amount of passivation agent required during manufacture, thereby providing an overall cost savings and a reduction to the back pressure associated with the operation of any given filter.